Nonvolatile semiconductor storage device

ABSTRACT

A flash memory  1  based on the multilevel storage technology for storing the information of two or more bits is provided with four banks  2   a  to  2   d . For example, in the left side of the bank  2   a , a data latch  6   a  is provided along one short side of the bank  2   a , while in the right side thereof, a data latch  6   b  is provided along the other short side of the bank  2   a . At the lower side of the data latches  6   a   , 6   b , arithmetic circuits  7   a   , 7   b  are provided. The data latches  6   a   , 6   b  are respectively formed of SRAMs. A sense latch  5   a  is divided to one half in the right and left directions with reference to the center of sense latch row. The divided sense latch  5   a  is connected with the data latches  6   a   , 6   b  via the signal lines respectively allocated along both short sides of the bank  2   a.

TECHNICAL FIELD

The present invention relates to a nonvolatile semiconductor storage device and particularly to technology which can be adapted effectively to reduction in chip layout area of a multilevel flash memory.

BACKGROUND ART

In recent years, semiconductor memories such as flash memories are in the trend that large capacity and low cost data are introduced more and more rapidly. As the technology corresponding to novel attempt in introduction of such large capacity data, a multilevel flash memory utilizing the multilevel storage technology has been very popular in which a plurality of threshold voltages of a certain voltage level are set depending on the amount of charges accumulated in a charge storage layer of a memory chip and the data of two bits or more is stored.

According to discussion by the inventors of the present invention, the multilevel flash memory is provided with a structure that a memory cell array is divided, for example, to about four banks. Each bank has a structure that a sense latch is provided at the center thereof and data latches for storing write data are also provided in the vicinity of the two longer sides of the bank. The sense latches hold the information of the sense operation and the write object cells, while the data latches store write data.

The Japanese Unexamined Patent Publication No. Hei 02 (1990)-246087 is an example of the document which describes in details layout technology of peripheral circuits in the semiconductor memory of this type. This document specifically describes the layout technology of a main amplifier provided within a DRAM.

However, the inventors of the present invention have found the following problems in the layout technology of sense latch and data latch in the multilevel flash memory as described above.

Namely, such a multilevel flash memory is accompanied by a problem that the layout area required for the data latch becomes wide because the data of two bits or more is transferred with only one memory cell and the chip area also becomes large to hinder reduction in size of the flash memory because a degree of freedom of layout is also restricted.

An object of the present invention is to provide a nonvolatile semiconductor storage device which can remarkably reduce chip area without deterioration of data transfer rate by optimizing layout of data latch.

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

DISCLOSURE OF THE INVENTION

The present invention is provided with a plurality of memory banks consisting of a rectangular shape memory array in which a plurality of threshold voltages of a certain voltage level are set and memory cells storing data of two bits or more in each cell are arranged in the form of an array, a sense latch allocated along one longer side of the memory bank to hold the information of sense operation and write object memory cells, first and second arithmetic circuits which are respectively allocated along one and the other short sides of the memory bank to perform multilevel arithmetic operation of the written data and determine a threshold voltage level to be written into the memory cell, and first and second buffers which are respectively allocated along one and the other short sides of the memory bank to store the write data.

Moreover, in the present invention, the input/output line connected to the sense latch is divided into two portions with reference to the center of the sense latch row, and one input/output line is wired along one short side of the memory bank, while the other input/output line is wired along the other short side of the memory bank.

In addition, the first and second buffers of the present invention are formed of SRAMs (Static Random Access Memories).

Moreover, the plural memory banks, sense latches, first and second arithmetic circuits and first and second buffers of the present invention are allocated over the semiconductor chip which is restricted in the length of the longitudinal direction depending on the package size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chip layout diagram of a flash memory as a preferred embodiment of the present invention;

FIG. 2 is a layout diagram of a bank, a sense latch, data latches and an arithmetic circuit of a memory mat provided to a flash memory of FIG. 1;

FIG. 3 is a circuit diagram of the data latches of FIG. 2;

FIG. 4 is a circuit diagram illustrating an example of the data latch circuit which the inventors of the present invention have discussed; and

FIG. 5 is a layout diagram of the signal lines for transferring the write data or the like to the sense latch provided to the flash memory of FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail with reference to the accompanying drawings. The like elements provided with the like functions are designated with the like reference numerals throughout the drawings for describing the preferred embodiment thereof for elimination of the repetitive description.

In the present embodiment, a flash memory 1 is a multilevel flash memory utilizing the multilevel storage technology for storing the information of two bits or more by setting a plurality of threshold voltages of a certain voltage level to one memory cell.

This flash memory 1 is formed, as illustrated in FIG. 1, of a memory array 2, a control circuit 3, a power supply circuit 4, sense latches 5, data latches 6, and arithmetic circuits 7.

In the flash memory 1, the control circuit 3 is provided along the peripheral portion in one longitudinal direction of a semiconductor chip CH. The control circuit 3 comprises a logic control circuit, an input/output control circuit, a command register, an address resister, and direct/indirect peripheral circuits such as column/row address decoders.

The logic control circuit temporarily stores control signals inputted from a host such as a microcomputer as the connection destination to control operation logic. The input/output control circuit controls input and output of signals such as external addresses and data inputted/outputted to/from the host.

The column/row address decoder decodes the signals on the basis of the column/row addresses. The control circuit controls the sense latches 5, data latches 6 and arithmetic circuits 7 under the control of the logic control circuit.

Moreover, the memory array 2 is allocated at the center of the semiconductor chip CH. In the memory array 2, memory cells which are formed as the minimum unit of storage are regularly arranged like an array.

This memory array 2 is divided into the four banks (memory banks) 2 a to 2 d to the lower side from the upper side to form a so-called multibank structure in which each bank 2 a to 2 d operates independently.

In addition, the power supply circuit 4 is provided in the peripheral portion in the other longitudinal direction of the semiconductor chip CH. This power supply circuit 4 generates a verification voltage used for the verification operation and various step-up voltages and step-down voltages used for data writing operation and also supplies these voltages to each of the banks 2 a to 2 d of the memory array 2.

The sense latches 5 are formed of four sense latches 5 a to 5 d and these sense latches 5 a to 5 d are respectively provided at the lower side of the banks 2 a to 2 d (along one longitudinal side of the banks 2 a to 2 d). These sense latches 5 a to 5 d hold the information of the sense operation and write object cells.

The data latches 6 are formed of data latches (first buffers) 6 a, 6 c, 6 e, 6 g and data latches (second buffers) 6 b, 6 d, 6 f, 6 h. These data latches 6 a to 6 h store the write data. The data latches 6 a to 6 h are respectively provided in unit of two latches in both right and left sides of each of the banks 2 a to 2 d. For example, in the case of bank 2 a, the data latches 6 a, 6 b are respectively provided in both right and left sides of the bank 2 a.

In the same manner, the arithmetic circuits 7 are also formed of eight arithmetic circuits 7 a, 7 c, 7 e, 7 g (first arithmetic circuits) and 7 b, 7 d, 7 f, 7 h (second arithmetic circuits). These arithmetic circuits 7 a to 7 h are respectively provided in unit of two circuits at the lower side of the data latches 6 a to 6 h in both right and left sides of the banks 2 a to 2 d. These arithmetic circuits 7 a to 7 h perform multilevel arithmetic operation of data written into the data latches 6 a to 6 h to determine a threshold voltage level to be written into the memory cells of the banks 2 a to 2 d.

Here, the layout of the sense latches 5, data latches 6 and arithmetic circuit 7 b will be described in detail.

FIG. 2 illustrates a layout of the sense latch 5 a, data latches 6 a, 6 b and arithmetic circuits 7 a, 7 b provided in the periphery of the bank 2 a of a memory mat 2. Moreover, the sense latches 5 b to 5 d, data latches 6 c to 6 h and arithmetic circuits 7 c to 7 h are also provided in the layout and circuit structure which are similar to that of FIG. 2.

As illustrated in FIG. 2, the data latch 6 a is provided along one short side of the bank 2 a in the left side of bank 2 a, while the data latch 6 b is provided along the other short side of the bank 2 a in the right side of bank 2 a.

In the same manner, the arithmetic circuit 7 a is provided at the lower side of the data latch 6 a in the left side of the bank 2 a, and the arithmetic circuit 7 b, at the lower side of the data latch 6 b in the right side of the bank 2 a.

Moreover, the data latches 6 a, 6 b are respectively formed of SRAMs. FIG. 3 illustrates an example of the circuit in which the latch circuit DR of the data latches 6 a, 6 b is formed of SRAMs.

As illustrated in the figure, the latch circuit DR is formed of the so-called six-transistor CMOS consisting of transistors T1, T2 and inverters V1, V2.

The data latches 6 a, 6 b are respectively formed of SRAMs which are arranged in the form of an array to store the data to be written or read with at least single write or read operation to or from the bank 2 a.

FIG. 4 is a circuit diagram illustrating a structure of the ordinary data latch circuit DR30 which the inventors of the present invention have discussed. In this case, the data latch circuit DR30 is formed of two inverters Iv30, Iv31 and five transistors Tr30 to Tr34.

As described above, the number of transistors can be remarkably reduced by forming the data latches 6 a, 6 b with the SRAMs. Since the layout area of the data latches 6 a, 6 b can be reduced, the semiconductor chip CH can be formed in small size.

Moreover, as illustrated in FIG. 5, the sense latch 5 a is divided to ½ in both right and left directions with reference to the center of the sense latch row. In this sense latch 5 a, the sense latch of the divided left side is connected with the data latch 6 a via the signal line SL1 as the common I/O, while the sense latch of the divided right side is connected with the data latch 6 b via the signal line SL2.

Since these signal lines SL1, SL2 are respectively allocated along both short right and left sides of the bank 2 a, namely along both short sides of the bank 2 a, the data can be transferred in parallel and the transfer rate can therefore be doubled.

Simultaneously, since the data latches 6 a, 6 b are allocated in both sides, the number of signal lines SL1, SL2 wired to the sense latch 5 a can be reduced to one half. Accordingly, the wiring regulations for the sense latch 5 a due to the signal lines SL1, SL2 can also be lowered.

In addition, in the flash memory 1 based on the multilevel storage technology, data transfer can be performed repeatedly between the data latches 6 a, 6 b and sense latch 5 a through the verification operation for the threshold voltage to check accuracy of the data during the write operation.

Therefore, higher data transfer rate can be realized through the parallel data transfer between the data latches 6 a, 6 b and sense latch 5 a and accordingly, the high speed write operation of the flash memory 1 can also be realized.

As a result, according to the present embodiment, since the layout area can be reduced remarkably by forming the data latches 6 a to 6 h with the SRAMs, the semiconductor chip CH can be reduced in size and can also be saved in chip cost.

In addition, the parallel data transfer can be achieved and the transfer rate can also be doubled by respectively allocating the signal lines SL1, SL2 in both right and left sides of the banks 2 a to 2 d.

Moreover, in the flash memory of this embodiment, the data latches are provided along the short side direction of the bank. However, when a vacant area is provided in the side of the short side direction of the bank, it is enough to allocate the data latches in the desired place of such vacant area.

Furthermore, in this embodiment, the flash memory is formed in the type of structure that a plurality of threshold voltages are set by controlling the amount of charges to be accumulated in the charge accumulation layer of a memory cell. It is also possible, however, to form the flash memory with the memory cells of the type that a plurality of areas are set locally to accumulate the charges to the charge accumulation layer of the memory cell.

Moreover, the multilevel flash memory is formed in this embodiment. However, it is also possible to introduce a binary level flash memory in which one-bit information is stored in one memory cell.

Although the preferred embodiment of the present invention has been disclosed, it is needless to say that various modifications, additions and substitutions are possible, without departing from the scope of the invention.

Industrial Applicability

As described above, the nonvolatile semiconductor storage device of the present invention is suitable for the technology for reduction in the layout area of semiconductor chip used in the multilevel flash memory. 

1. A nonvolatile semiconductor storage device in which a plurality of threshold voltages of a certain voltage level are set and the data of two bits or more is stored in each of memory cells, comprising: a plurality of memory banks, each of which is comprised of a rectangular memory array in which said memory cells are arranged in the form of an array; sense latches which are allocated along one longer side of said memory banks to hold the information of sense operation and said memory cells as the write object; first arithmetic circuits which are allocated along one short side of said memory banks to determine a threshold voltage level to be written to said memory cells by performing multilevel arithmetic operations of the written data; second arithmetic circuits which are allocated along the other short side of said memory banks to determine a threshold voltage level to be written to said memory cells by performing multilevel arithmetic operations of the written data; first buffers allocated along one short side of said memory banks to store the written data; and second buffers allocated along the other short side of said memory banks to store the written data.
 2. The nonvolatile semiconductor storage device according to claim 1, wherein an input/output line connected to said sense latch is divided to one half with reference to the center of sense latch row of said sense latch, one input/output line is allocated along one short side of said memory bank, and the other input/output line is wired along the other short side of said memory bank.
 3. The nonvolatile semiconductor storage device according to claim 1, wherein said first and second buffers are respectively comprised of SRAMs.
 4. The nonvolatile semiconductor storage device according to claim 1, wherein a plurality of said memory banks, sense latches, first and second arithmetic circuits and first and second buffers are provided, within a package size, over a semiconductor chip in the layout restricted in the length of the longitudinal direction.
 5. The nonvolatile semiconductor storage device according to claim 2, wherein said first and second buffers are respectively comprised of SRAMs.
 6. The nonvolatile semiconductor storage device according to any one of claim 5, wherein a plurality of said memory banks, sense latches, first and second arithmetic circuits and first and second buffers are provided, within a package size, over a semiconductor chip in the layout restricted in the length of the longitudinal direction.
 7. The nonvolatile semiconductor storage device according to any one of claim 2, wherein a plurality of said memory banks, sense latches, first and second arithmetic circuits and first and second buffers are provided, within a package size, over a semiconductor chip in the layout restricted in the length of the longitudinal direction.
 8. The nonvolatile semiconductor storage device according to any one of claim 3, wherein a plurality of said memory banks, sense latches, first and second arithmetic circuits and first and second buffers are provided, within a package size, over a semiconductor chip in the layout restricted in the length of the longitudinal direction. 